1. Field of the Invention
The present invention relates to a high frequency switching circuit used to switch a transmission path of a high frequency signal in a mobile communication apparatus and a wireless LAN.
2. Description of the Related Art
In a mobile communication apparatus, such as a cellular phone, a transmission circuit and a reception circuit share one antenna. In general, a high frequency switching circuit for switching an antenna is provided between the antenna and the reception and transmission circuits. The high frequency switching circuit switches the antenna between the transmission circuit during the transmission and the reception circuit during the reception.
FIG. 4 is a view illustrating a high frequency switching circuit according to the related art, which is disclosed in Japanese Patent Laid-Open Publication No. 2006-042138.
A high frequency switching circuit 10 includes high frequency switches 16 and 17 that are composed of semiconductor FETs. The high frequency switch 16 is connected between a transmission input terminal 11 and an antenna terminal 12. The high frequency switch 17 is connected between a reception input terminal 13 and an antenna terminal 12.
The high frequency switch 16 is turned on and the high frequency switch 17 is turned off during the transmission according to control signals applied to control terminals 14 and 15.
During reception, the high frequency switch 16 is turned off, and the high frequency switch 17 is turned on.
As shown in FIG. 4, a large-power amplifier 20, a filter 30, and a phase shifter 40 are connected to the transmission side of the high frequency switching circuit 10. A high frequency signal is supplied to the transmission input terminal 11.
The reception side receives the high frequency signal through a low noise amplifier 50 that is connected to the reception input terminal 13.
Further, an antenna 60 is connected to the antenna terminal 12.
In the high frequency switching circuit, a switch with low distortion to reduce high-order harmonics even when a large signal is input is required as a high frequency switch, especially for the high frequency switch 16 that is used in the transmission side.
In order to meet these needs, FETs forming the high frequency switches may be connected in multi stage or have a multi-gate structure, a boost circuit is provided to turn on or off the high frequency switches, or a feedback capacitor circuit is additionally provided to the high frequency switches. In this way, the high frequency switches with low distortion are implemented.
Even when the high frequency switches with low distortion are used, high-order harmonics caused by the high frequency switches are significantly changed due to the impedance of devices connected before and after the high frequency switches, such as an antenna, a filter, and an amplifier.
In the configuration disclosed in Japanese Patent Laid-Open Publication No. 2006-042138, shown in FIG. 4, a phase of the high frequency signal, which is input to the transmission input terminal 11, is adjusted by the phase shifter 40 provided at the transmission side of the high frequency switching circuit 10.
The phase or an electrical length of a connection line between the high frequency switching circuit 10, the large-power amplifier 20, and the filter 30 provided at the transmission side is adjusted to thereby optimize characteristics.
However, in the configuration of FIG. 4, inductances or capacitances in the phase shifter 40 need to be separately and individually adjusted for the optimization with respect to imbalanced impedance characteristics or kinds of individual devices connected to transmission input terminal 11, which increases manufacturing costs.